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BEHAVIOUR AND DESIGN OF LOW DENSITY AIRCRETE MASONRY
M C Limbachiya*, C A Fudge+, J J Roberts**
* Professor, Faculty of Engineering, Kingston University- London, UK +
Aircrete Products Association, UK
** Emeritus Professor, Faculty of Engineering, Kingston University- London, UK
SUMMARY
This paper outlines part of a comprehensive research project carried out by Kingston
University’s Concrete and Masonry Research Group to assess the behaviour of high
performance low-density aircrete masonry, with a declared compressive strength of 2.0N
(N/mm2) and a density of 350 kg/m
3. Performance was then compared with the conventional
aircrete masonry blocks with a declared compressive strength of 2.9N and a density of 460
kg/m3. The first part of the paper concentrates on the fundamental characteristics of low
density aircrete (LDA) masonry units, including compressive strength, dimension, density,
moisture properties, thermal performance, freeze/thaw resistance. Thereafter, the structural
performance of key ancillary components used with low-density aircrete masonry is examined
and discussed. In addition, practical issues associated with the use of low density masonry
units are also highlighted.
INTRODUCTION
Aircrete originated from Scandinavia in the 1920’s and is now used extensively in the UK,
with the current European market being over 30 million m3. At present, sales of aircrete
masonry units are approximately 3 million m3 per annum in the UK and, as blocks they are
used extensively by house builders, although there are some sales to other sectors of the
construction industry. It is so extensively used that Aircrete accounts for approximately a
third of all concrete masonry units used in the UK. Aircrete blocks are suitable for use as
load-bearing and non load-bearing masonry walls and contribute significantly towards the
thermal insulation required from wall construction. Aircrete units may also be used as outer
leaves of masonry external cavity walls, solid external walls, internal partitions and separating
walls between dwellings, and walls below ground level.
The main benefit is that acoustic, energy conservation, fire resistance and structural properties
are uniquely provided in one product. The lightweight products have very low thermal
conductivities, making them ideal materials for external walls where heat loss is of primary
importance. Despite their relatively low mass, aircrete products perform well where acoustic
performance is required, due to the micro-structure of the material.
There have been considerable developments in product properties and construction methods,
which have accelerated in recent years, making available higher strengths, lower densities and
larger size units than previously. Innovative methods of construction have also been made
possible as a result of modern manufacturing techniques e.g. thin layer mortar jointing and
blocks with hand holds in the block perpends.
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KEY CHARACTERISTICS CONSIDERED
Compressive Strength
It is widely recognised and accepted that the compressive strength of aircrete is related to its
density and increases with increasing density. Commonly produced compressive strengths are
2.9, 3.6, 7.3 and 8.7N (N/mm2) as indicated in Table 1. In the UK compressive strengths > 4
N/mm2 are commonly used, however, in Europe lower strength Aircrete has been successfully
utilised for the construction of dwellings (Wittmann 1993), implying lower compressive
strength could be adequate. The compressive strength of Aircrete is nearly independent of
specimen size due to its homogeneity (Wittmann 1993). Aircrete achieves its final strength
during the autoclaving process without further curing being necessary.
Table 1 Physical Properties of Aircrete Blocks
Aircrete Density
Compressive
Strength
(N/mm2)
Density
(kg/m3)
Thermal
Conductivity
(W/mK)
Low 2.0 – 3.6 450 0.09 - 0.11
Medium 3.6 – 4.5 620 0.15 – 0.17
High 7.0 – 8.5 750 0.19 – 0.20
Thermal Resistance
Thermal conduction is the phenomenon by which heat is transported from high to low-
temperature regions of a substance. The high degree of porosity of Aircrete has a dramatic
influence on thermal conductivity; increasing pore volume will, under most circumstances
reduce thermal conductivity and increase thermal insulation. Heat transfer across pores is
ordinarily slow and inefficient (Callister 2000). Internal pores normally contain still air, which
has extremely low thermal conductivity - approximately 0.02 W/m-k. Furthermore, gaseous
convection within the pores is also comparatively ineffective. Hence Low Density Aircrete
has outstanding thermal insulation properties (Dubral 1992, Theramalite 2003, Limbachiya
and Roberts 2005, Fudge and Limbachiya 2006, Schlegel and Volec 1992) as given in Table 1
above.
Moisture Movement and Resistance to Freezing
Moisture movement through porous building materials is a very complex process and for
practical predictions simplifying assumptions may have to be introduced. There are at least
three different origins of water in aircrete masonry units. Immediately after autoclaving
Aircrete contains typically about 30% water by weight of the dry material. This excess water
is lost under normal conditions to the surrounding air after a few years (Dubral 1992). If the
relative humidity of the surrounding air increases temporarily, aircrete will take up water
again by absorption and capillary condensation. If the surface of a structural element is in
contact with liquid water the material absorbs water quickly by capillary suction (Mitsuda and
Kiribayashi 1992, Wittmann 1993). Although low density aircrete units has a very high
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proportion of air, the pores are fine and are not interconnected, therefore, the material offers
good resistance to moisture penetration.
Aircrete possesses good resistance to freezing, which is proved by un-rendered buildings,
situated in areas where frequent freeze/thaw cycles occur, remaining undamaged. The reason
for the good resistance is that the included spherical pores are almost all closed, meaning the
material has comparatively low capillary suction and therefore the moisture content does not
normally reach the critical degree. Moreover, as the pores are not interconnected, this
radically reduces the possibility of water absorption. The high freeze-thaw resistance in
essence is due to the aerated internal structure of the material. The resistance to frost is
superior to that of many stronger denser masonry materials although the degree of resistance
is to some extent dependent on strength, as well as density.
EXPERIMENTAL PROGRAMME
Given that the efficiency of construction is improved by providing masonry units that are
easily manhandled and readily cut, shaped and chased, such as LDA units, there is the
potential for simplified external wall construction, which address several of the key aspects of
environmentally friendly products. The Concrete and Masonry Research Group at Kingston
University undertook extensive research aimed at assessing the behaviour and design of high
performance LDA products. The principle objective of the research was to create further
value-added outlets for exploiting the environmentally friendly and beneficial properties of
high performance low-density aircrete in dwellings in the UK. This paper outlines standard, as
well as routine test methods, used to assess thermal characteristics of aircrete masonry units,
as well as structural performance of key ancillary components used with low-density aircrete
blocks. The declared densities of the 2.0 and 2.9N/mm2
were 350 and 460kg/m3 respectively.
As a starting point, dimension, density and moisture properties of test samples were
established. Dimension and Density of aircrete masonry units were determined following the
procedure described in BS EN 772: Parts 16 and 13, respectively. Moisture properties of low
density aircrete masonry samples were also established by measuring the moisture content,
water absorption and movement. A minimum of six representative portions from at least three
units were tested. For this, after drying to constant mass, the moisture content was calculated as
the ratio of the loss of mass during drying to the mass after drying. The moisture content was
also measured immediately after removing the blocks from the pallets. After drying to constant
mass a face of the Aircrete block was immersed in water for a specific period of time, to
evaluate the coefficient of water absorption at 10, 30 and 90 minutes. The results of this test
series are given in the paper presented at the 7th
International masonry Conference in 2006,
organised by the British Masonry Society (Limbachiya and Fudge, 2006).
Thermal Performance
The thermal performance of LDA masonry units was verified by reference to BS EN 1745:
2002, which incorporated testing conforming to BS EN 12664: 2001 using a guarded hot-
plate. Thermal conductivity testing, in accordance with BS EN 12664: 2001 was carried out
by H + H UK. The results were expressed to the nearest 0.1 W/mK. For this, guarded hot-
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plate apparatus in a vertical orientation conforming to the appropriate standard was used. The
guarded hot-plate had;
•••• Heating unit, which consists of guarded section heater surface
plates;
•••• Cooling unit, which consists of cooling unit surface plates;
•••• Thermocouples consist of heating and cooling unit surface
thermocouples, respectively.
A representative specimen of 305 x 305 x 50 mm was prepared from 2.0 N/mm2 aircrete
specimens of 300 x 620 x 200 mm. The faces of which were milled to produce a specimen
thickness of 45 mm with plane and parallel surfaces using a diamond toothed rotary milling
machine. The difference in thickness across the full width was maintained at less than 2% of
the mean thickness (< 0.9mm) and deviation from flatness was less than 0.08mm over the full
width.
Structural Performance of Ancillary Components
Lintel Bearings
During this research, a range of proprietary steel lintel bearings, joist hangers, wall ties and
fixings were tested to evaluate the structural performance of low density aircrete masonry
units- in wallettes prepared with thin joint mortar. The material preparation and testing were
carried out in accordance to BS EN 846. Lintels were tested (Figure 1) at bearing lengths of
150 and 300mm, and it was subjected to a uniformly distributed load (4 – point). The mid–
span vertical deflection, together with any visible signs of distress in specimens, fixings or
supporting member, was recorded.
Figure 1 Lintel Bearings Experimental Test Sep-up
LOAD
Aircrete Wallettes
Bearing
length
Steel Lintel
Deflection
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Joist Hangers
Joist hangers testing was conducted to ascertain the compatibility of using joist hangers with
LDA masonry. The bearing area of the hanger was designed to work with 2.0 and 2.9N
Aircrete masonry. Walls of the low strength masonry were constructed and the hangers and
joist were incorporated as specified in the test procedure BS EN 846: Part 8: 2000. Stainless
steel joist hangers were fixed to a wall (constructed from 2.0N and 2.9N aircrete masonry)
and loaded through joists (timber). After construction (with thin layer mortar), the wall
specimens were covered in polyethylene sheets and cured for 28 ± 1 day before testing. For
all the tests, the timber length was 1m. The loading system applied a vertical load to the
specimen. The maximum loads and the mode of failure for all the specimens were recorded.
As the load was applied at a distance of 2L / 3 from the joist hanger, the force sustained by
each individual hanger would be two – thirds of the maximum failure load. The maximum
load value for the timber joist was the value sustained by the timber joist at a distance of 2L/ 3
from the joist hanger, which is the point A, as shown in Figure 2. A pre-load of 1 kN was
applied to the test specimen and held for a period of 1 minute. The load was then removed and
a load was applied at a rate of 1 kN increase per minute until failure occurred, which was
defined as the load at which further deflection occurs without increase in test load. Three joist
hangers, positioned at 1, 2 and 3 (from left to right, Figure 3) were used for this testing. The
failure load and mode of failure was recorded, and maximum load sustained by the joist hanger
was calculated as failure load/3.
Figure 2 Schematic Representation of Test (Force sustained by joist hanger = X/3)
Figure 3 Typical wallette specimen and joist hanger test set-up
LOAD = X
2L / 3
L
A Joist hanger
Timber joist
1
2
3
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RESULTS AND DISCUSSIONS
Dimension and Density: The test results for dimensions and density are summarised in Tables
2 and 3, respectively. Whilst average values are given in Table 4 below.
Table 2 Dimensions of 2.0 and 2.9N Aircrete Masonry Units
Aircrete Length
(mm)
Width
(mm)
Height
(mm)
2.0
N
620.0 149.8 249.9
619.5 149.6 249.6
620.0 150.0 250.0
620.0 149.8 249.7
619.8 150.0 250.0
619.6 149.4 249.4
2.9
N
440.0 149.6 214.5
439.6 150.0 215.0
439.6 149.7 214.5
440.0 149.8 214.8
439.8 150.0 215.0
439.5 149.5 214.8
Table 3 Density of 2.0 and 2.9N Aircrete Masonry Units
Aircrete Density (kg/m3)
2.0
N
350
352
354
351
352
353
2.9
N
474
475
477
479
480
477
Table 4 Average Dimensions and Density of 2.0 and 2.9N Aircrete Units
Aircrete
Average values
Length
(mm)
Width
(mm)
Height
(mm)
Density
(kg/m3)
2.0 N 619.8 149.8 249.8 352
2.9 N 439.7 149.8 214.8 477
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Dimensions given by manufacturers for both aircrete masonry units used were:
• For 2.0N aircrete masonry units- 620 x 150 x 250 mm
• For 2.9N aircrete masonry units- 440 x 150 x 215 mm
On comparison, average measured dimensions for both aircrete units were found to be within
0.3mm of declared dimensions given by the manufacturers. The average values are also
within the permissible deviations for use with conventional mortar and thin layer mortar as
specified in accordance with BS EN 998: Part 2: 2003. Declared manufacturer’s density for
2.0 and 2.9N aircrete unit is 350 kg/m3 and 475 kg/m
3, respectively. Against this, average
measured density for 2.0 and 2.9 N/mm2 Aircrete were 352 kg/m
3 and 477 kg/m
3 respectively,
within 0.57% and 0.42% of the theoretical density value.
Thermal Performance
Results of thermal conductivity tests are summarised in Table 5 below.
Table 5 Thermal Conductivity Test Results (2.0 N/mm2 aircrete masonry units)
Thermal
conductivity
(W/mK)
Moisture content
% (w/w)
Dry
density (kg/m3)
Mean
temperature at
test (°C)
Density of heat
flow rate
(W/m2)
0.104 3.55 339.1 20.3 70.07
The test result shows that 2.0N Aircrete unit has a very low thermal conductivity of 0.104
W/mK and lower than 2.9N Aircrete (0.11 W/mK). This was expected considering the lower
density of 2.0N Aircrete due to its higher porosity as compared to 2.9N Aircrete. The Low
Density Aircrete of 2.0N unit has a relatively higher porosity of 70 -85% as compared to
ordinary Aircrete, which has 60 – 85% porosity. The high degree of porosity of 2.0N Aircrete
masonry unit has drastically improved the thermal resistance.
Structural Performance of Ancillary Components
Lintel Bearing
Lintels bearing were tested at bearing lengths of 150 and 300 mm on 2.0N and 2.9N aircrete
wallettes and the test results are summarised in Table 6. Results show that all wallettes can
withstand at least a load of 75 kN or a stress value of 1.7 N/mm2. Furthermore, there was no
visible failure and on removal of the load, the deflection reading went back to zero, hence,
elastic deflection took place. For 2.0N Aircrete wallette at 150mm bearing length, the
deflections were higher than that of 2.9N Aircrete wallette. Also, the deflections were larger
in magnitude at 150mm bearing length. The walls, therefore can withstand higher loads,
however, due to health and safety implications, the testing was stopped at a load of 75 kN.
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Table 6 Lintel Bearing Test Results for Different Aircrete and Mortar Combination
Aircrete
Bearing
length
(mm)
Max. load
(kN)
(stress
N/mm2)
Any
visible
failure
Deflection
at 75 kN
(mm)
Deflection
upon load
release (mm)
2.0 N 150 75 (1.7) NO 7.8 0
300 75 (0.8) NO 5.6 0
2.9 N 150 75 (1.7) NO 6.5 0
300 75 (0.8) NO 6.0 0
Note: Thin layer mortar was used in test wallettes production
Joints hangers
The maximum load recorded from the test and the mode of failure for all the specimens are
given in Table 7. Figure 4 shows the typical failure mechanism for all the tests. In all cases
the timber joists failed, whilst all the 6 wallettes specimens remained totally unscathed. The
test results were very consistent with the timber joist failing at loads between 16.1 – 17.5 kN,
which equates to the joist hangers being able to sustain at least a minimum load of 5.4 kN.
Table 7 Recorded Failure Loads from the Tests
Figure 4 Typical Failure for all Tests Showing Timber Joist Failure
Aircrete
Maximum Load, kN
Mode of Failure Timber
joist
Joist @ positions
1 2 3
2.0 N 16.6 – 16.8 5.5 5.6 5.5 Joist
2.9 N 17.4 – 17.5 5.8 5.8 5.8 Joist
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Table 8 gives the test results of testing of metal joist, instead of timber joist on 2.0N Aircrete
wallette with thin layer mortar to evaluate the absolute maximum load capacity of the joist
hanger. The failure mode of joist hangers was found to be localised with crushing at the front
edge of the wall with no visible crack on the wall and the joist hanger was still intact to the
wall. This is possibly due to the restrain imposed by design of the joist hanger. Hence, the
recorded maximum load of 8.0 kN shown in Table 8 represents the serviceability limit of the
hanger and not the ultimate limit.
Table 8 Recorded Failure Loads of Metal Joist Test
Aircrete Types of
mortar
Timber
joist
Joist hanger @ position
(maximum load, kN) Mode of
failure 1 2 3
2.0 N Thin layer 24.0 - 8.0 - Joist hanger
CONCLUSIONS
•••• Dimension, density and moisture properties of low-density aircrete masonry units
were assessed using different standard test methods. The average measured
dimensions for both 2.0N and 2.9N LDA masonry units were within 0.3mm of the
declared values. Whilst the average measured density for 2.0N and 2.9N Aircrete unit
was 350 kg/m3 and 475 kg/m
3, respectively, which is noticeably within 0.57% and
0.42% of the theoretical or desired value.
•••• The thermal performance test results show that 2.0N LDA masonry has a very low
thermal conductivity of 0.10 W/mK.
•••• Lintel bearing testing was conducted to ascertain the load bearing capacity for both
2.0N and 2.9N LDA wallettes incorporating thin layer mortar. Test results confirmed
that all wallettes can withstand a load of 75 kN or a stress of 1.7 N/mm2 without any
visible signs of failure. Moreover, the deflections observed for the steel lintel during
testing were totally elastic.
•••• The compatibility of using a generic steel joist hanger with 2.0N and 2.9N aircrete has
been ascertained using thin layer type mortar. In all tests, the timber joist (with
dimensions of 100 x 38 x 1000 mm) failed before the masonry wallette, in which each
of the masonry specimens was left totally unscathed. Furthermore, the average
maximum load sustained by the timber joist during testing was 16.9 kN, and the
serviceability limit of the joist hanger was found to be 8 kN.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the EPSRC, Aircrete Products Association, H+H UK,
Hanson-Thermalite, Quinn-Group and Tarmac Topblock, Building Research Establishment,
Catnic- Corus UK, National House-Building Council and Department of Communities and
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Local Government for funding and supporting the work reported. Acknowledgement is also
given to the significant contributions made by the project Steering Panel members, namely
Dennis Coward, Alan Ferguson, Trevor Grounds, Jim Holland, Chris Kirby, Paul Matthews,
Gopal Sangarapillai and Richard Shipman.
REFERENCES
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aerated concrete units. 1999
� BS EN 772, Part 11– Determination of water absorption of aggregate concrete, manufactured
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absorption of clay masonry units, 2000.
� BS EN 771, Part 4– Specification for masonry units, Autoclaved aerated concrete masonry
units, 2003.
� BS EN 680– Determination of the drying shrinkage of autoclaved aerated concrete, 1994.
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